It is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473, 4,115,128, 4,173,470 and 4,550,069. These include alkali-soluble novolak resins together with light-sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired.
The novolak or polyvinyl phenol resin component of these photoresist formulations is soluble in aqueous alkaline solutions, but the naphthoquinone sensitizer acts as a dissolution rate inhibitor with respect to the resin. Upon exposure of selected areas of the coated substrate to actinic radiation, however, the sensitizer undergoes a radiation induced structural transformation and the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate.
An important group of photosensitizers are the condensation products of 1,2-naphthoquinonediazide-4-sulfonic acid or 1,2-naphthoquinonediazide-4-sulfonic acid and polyols. These compounds tend to exhibit superior photospeed and contrast in the mid-UV region of the light spectrum when formulated in photoresist compositions.
The trend in microlithography during the past few years has been an accelerated drive towards smaller geometries. The efforts to achieve these shrinking design rules require intensive efforts by both exposure tool and photoresist manufacturers. Therefore there is an increasing demand for UV-2 (DEEP-UV) and UV-3 (I-LINE) sensitive photoresist systems. Novolak containing photoresists are still workable for g-line and i-line type resists. 2,1,5-diazonaphthoquinone sulfonate esters of trihydroxy benzophenones are generally used as photoactive compounds (PAC) for broad band or g-line resists. On the other hand 2,1,4-diazonaphthoquinone esters are more suitable for i-line (365 nm) region.
Synthesis of such diazonaphthoquinone esters is normally conducted in a solvent system such as: N-methyl pyrollidone (NMP), acetone, acetonitrile or mixed solvent system. A base is generally used as a catalyst and also as an acid acceptor. Common organic bases used, while not inclusive, include pyridine, triethylamine, N-methyl morpholine, dimethyl amino pyridine and mixture thereof. The processes utilizing these solvent/base systems provide diazonaphthoquinone esters of consistently good quality. However, providing similar quality and consistency of the diazo esters becomes extremely difficult if a similar process is utilized for making 2,1,4-diazo esters or 2,1,4/2,1,5 mixed diazo esters. In addition, making 2,1,4 diazo esters or 2,1,4/2,1,5 mixed diazo esters becomes even more difficult when utilizing a tetrahydroxybenzophenone.